1. Field of the Invention
The present invention relates to a regenerable electric capacitor, and more particularly to such a capacitor for a.c. voltage applications which comprises wound-on layers of synthetic foils which have been provided with metal coatings, where metal-free edge strips are arranged along the longitudinal sides of the foils, and to a process for the production of such capacitors.
2. Description of the Prior Art
Capacitors of the general type discussed above are well known in the art. At high field strengths, capacitors heretofore known generally possess capacitance values which decrease with increasing operating duration. The reason for this is that the vapor-deposited metal coatings exhibit characteristic changes during continuous operation on the overshooting of an a.c. field strength which is dependent, for example, upon the nature of the foil, the thickness of the foil and the coating metal. This is due to the fact that with the low coating thickness, due to the high edge field strengths which have 10 to 100 times higher values than the homogenous field strength within the capacitor, in the presence of moisture and/or oxygen, he aluminum metal coating is transformed, starting from the edges of the coating and the pores in the coating surface to form highly insulating aluminum oxide and/or aluminum hydroxide. The speed at which the coating disintegrates, therefore, increases in proportion to the content of water vapor and/or oxygen in the atmosphere with which the capacitor comes into contact. In order to overcome these difficulties, it is known to dry the capacitors referred to above in a vacuum at increased temperatures and to protect the capacitors from the entry of water vapor and/or oxygen by means of careful installation, such as, for example, casting the capacitors in epoxide resin.
It is known to protect the capacitor installed in a housing by at least partially filling the capacitor container with substances which absorb oxygen and/or water vapor. In this connection, one may refer to the German published application No. 2,415,314.
Furthermore, it is known in the art to fill the capacitor housing with a suitable insulating liquid, as disclosed, for example, in the German published application No. 2,539,781, which corresponds to U.S. Pat. No. 3,987,348.
In addition, the German published application No. 2,359, 431 discloses a capacitor in which the high edge field strength is disintegrated in that an edge strip composed of a metal, in particular aluminum, having a thickness of less than 25 nm, is applied, and is subsequently converted into a highly ohmic compound by an electrochemical and/or chemical and/or temperature process in the presence of oxygen and/or water. This process is not very suitable for regenerable capacitors as the danger exists that the entire coating will be come oxidized during the treatment.